This invention relates to a retractible stop which moves between a retracted position and an operative position in which it temporarily blocks the motion of an article which is being urged to move along a conveyor system, or the like. This invention is particularly concerned with the problems created when the blocked article tends to exert sufficient force on the stop, in the operative position of the latter, to inhibit unduly the retractile motion of the stop when such motion is called for.
The problem under discussion can occur in any automatic system for conveying articles from position to position, and retaining them temporarily in a blocked position. For example, the problem has been encountered in conveyor systems used to transport vial-carrying trays in a nuclear counting instrument. In such an instrument, samples carried in vital trays are moved into position for elevation or lowering into a counting chamber. When the vital trays are stopped by contact with a stop element in its operative position, the tray-moving belts may continue to move by slipping on the bottom surfaces of the trays. In some cases it is necessary to move one row of trays while holding another row of trays stationary by means of stop elements. In such cases, the belts may be required to slip under a full row of trays, thereby causing a significant force by the stopped trays against the stop elements which they engage. This force may interfere with the desired retraction of the stop elements, which is customarily accomplished by energizing solenoids.
In the simplest form of solenoid-retracted stop element, the stop element is a pin, or post, extending from the solenoid armature. In the structure illustrated in FIG. 1, which is shown to illustrate the problems encountered, the solenoid-retracted pin extends through a bushing, which guides the pin and prevents lateral displacement of the pin under the force of the tray, or other article, which is being retained in position by the pin. The fractional resistance of the bushing against retractile movement of the pin adds to the force requirements of the solenoid.
One of the most important considerations in certain types of instrumentation is limiting the space required by the components of the apparatus. It is therefore desirable to permit a relatively small solenoid to control the movement of the stop element, avoiding the use of a "brute force" approach. Also it is important to minimize the production cost of the device, its complexity, and its power requirements.
Certain prior art structures have utilized conveyor release, or retractible stop, device in which the force exerted by the blocked article provides a retraction-assisting force on the device under certain conditions, such as those disclosed in Laughter U.S. Pat. No. 2,926,815 and in Schlottmann et al U.S. Pat. No. 3,970,180. However, those prior art structures incorporate latching and unlatching arrangements and their configurations are much too complex, bulky and costly to solve the problems to which the present invention is addressed.